Conventionally, Cathode Ray Tubes (CRTs), Electroluminescence (EL) displays, Plasma Display Panels (PDPs) etc. have been put into practical use as light emissive type displays, in which the content of the display can be overwritten electrically.
However, these types of light emitting displays generally have high power consumption. Further, the light-emitting surfaces of these types of displays are highly reflective. Therefore if the display is used under circumstances where the ambient light is brighter than the luminance (for example, in direct sunlight), then a phenomenon known as “wash-out” invariably occurs, and the display cannot be easily observed.
On the other hand, LCD devices have been put into practical use as non-light emissive type displays. That is, LCD devices display characters and/or images by using a background light source rather than by emitting a display light. These LCD devices include a transmission type LCD device and a reflection type LCD device.
Of the two types of LCD devices, the transmission type is more popular. The transmission type LCD device employs a light source called a “backlight” behind the liquid crystal cell. Since transmission type LCD devices are advantageous due to their thinness and light weight, they have been used in numerous, diverse fields. However, transmission type LCD devices consume a large amount of power to keep the backlight on. Thus, even though only a small amount of power is consumed to adjust transmittance of liquid crystals of the LCD device, a relatively large amount of power is consumed overall.
Transmission type LCD devices wash out less frequently compared with light emissive displays. In particular, in the case of color transmission type LCD devices, the reflectance on the display surface of a color filter layer is reduced by reflectance reducing means such as a black matrix.
It becomes difficult to readily observe the display on color transmission type LCD devices when they are used under circumstances where the ambient light is very strong and the display light is relatively weak. This problem can be mitigated or eliminated by using a brighter backlight, but this solution further increases power consumption.
Unlike light emissive displays and transmission type LCD devices, reflection type LCD devices show the display by using ambient light. Thereby a brightness of the display is proportional to the amount of ambient light. Thus, reflection type liquid crystal displays are advantageous insofar as they do not wash out. When used in a very bright place in direct sunlight, for example, the display can be observed all the more sharply. Further, the reflection type liquid crystal display does not use a backlight, and therefore has the further advantage of low power consumption. For the above reasons, reflection type LCD devices are particularly suitable for outdoor use, such as in portable information terminals, digital cameras, and portable video cameras.
However, since reflection type LCD devices use ambient light for the display, the display luminance largely depends on the surrounding environment. When the ambient light is weak, the display cannot be easily observed. In particular, in the case where a color filter is used for realizing the color display, the color filter absorbs much light and the display is darker. Thus, when the LCD device is used under these circumstances, the ambient light problem is even more pronounced.
Therefore, what is needed is a transflective LCD which can overcome the above-described problems.